External hard drive connected to the USB port of an AirPort Extreme Base Station (802.11ac) on your networkIf your backup drive isn't connected using one of these methods, Time Machine might not work. Check with the device manufacturer for compatibility and connection information.

If your setup meets these requirements, but you still can't use Time Machine, check these things:

Check your AirPort base station

Restart your base station and see if the issue continues. To restart, unplug the base station from AC power for 5 seconds, then plug it back in.

Check your network connection

If you back up to a server, AirPort Time Capsule, or drive connected to your AirPort Extreme Base Station, make sure that your Mac is connected to the same network as your backup drive. You can select your wireless network from the Wi-Fi status menu.

Check your backup

If you back up to a server, AirPort Time Capsule, or drive connected to your AirPort Extreme Base Station, make sure that your current Time Machine backup doesn't have any issues that could prevent further backups:

While holding down the Option key on your keyboard, click the Time Machine menu in the menu bar, then choose Verify Backups.

If Time Machine finds an issue with your backup, it displays a message with details. Follow the onscreen instructions.

Check your drives

If you're using a drive connected to a port on your Mac or AirPort Extreme Base Station, make sure that the drive is turned on.

If you're using a USB hub or a similar device, try connecting the drive directly to your Mac or base station.

If you're backing up to an external third-party drive, check with the drive manufacturer to make sure the drive's firmware is up to date.

If you're backing up to an external drive, check the formatting of the drive. Time Machine requires that an external drive be formatted like a Mac startup disk: Mac OS Extended (Journaled) with a GUID Partition Table (GPT). If you select a differently formatted drive for use with Time Machine, your Mac automatically prompts you to erase it for Time Machine.Reformatting a disk erases all data stored on the disk, so you might want to move important files to another disk first.

If your drive is already formatted correctly, turn off Time Machine, then check it for problems:

To check an external drive connected to your Mac, open Disk Utility from the Utilities folder of your Applications folder. Then use the First Aid feature in Disk Utility to verify your disk. Do this for your startup disk as well.

You can turn Time Machine back on after your drives are verified or repaired successfully.

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